Human Vibration Calibration: How Portable Shakers Measure Up

Until now most of the testing and calibration requirements within ISO 8041: “Human response to vibration – measuring instrumentation” have been outside the realm of possibility for the Industrial Hygienist. But The Modal Shop Models 9200D and 9210D Low Frequency Portable Vibration Calibrators bring revolutionary field testing capability to the Health & Safety professional, allowing them to meet not only the “in-situ” requirements of ISO 8041 but also mechanical “verification testing” requirements. Larson Davis’ HVM200 and SEN027 seat pad accelerometer make calibration simple by allowing the user to remove the accelerometer from the pad itself rather than apply a bulky, high-transverse adaptor.

There are three levels of calibration requirements within ISO 8041: pattern evaluation, verification testing and in-situ checks:

Pattern evaluation is full testing, targeted at human vibration meter and sensor manufacturers. It ensures the meter design meets the ISO standard. This aspect of the standard does not apply to the user.

Periodic verification testing is an intermediate set of tests geared toward end users and manufacturers. Per ISO 8041 they should be performed “every 1 or 2 years…to verify that the (instrumentation) performance remains within the specifications.”

In-situ checks are specifically targeted at users, encouraging a minimum level of testing before and after measurements are made to ensure quality data.

Available for rental or purchase, Models 9200D and 9210D are the first and only Portable Calibrators to meet – or come close to meeting – several key testing and calibration requirements within ISO 8041. Specific test clauses to which these calibrators can be applied are listed below. To learn more click on each clause to expand and read detailed analysis regarding how The Modal Shop shakers “measure up.”

Mechanical testing requirements (requirements imposed upon the shaker) state mechanical input signals must be within +/- 3% of the required value and total distortion must not be greater than 5%.

The Modal Shop is 9200D/9210D shakers meet or exceed the accuracy and distortion requirements from 2 Hz to 2 kHz. Hence the devices meet or exceed quality requirements for mechanical testing in hand-arm and whole-body vibration. Distortion is too high at frequencies near those required for low-frequency whole-body calibration.

This test is a simple % error calculation mechanically comparing the known vibration input from a shaker table to the displayed vibration on the meter itself. For hand-arm applications the reference test point is 10 m/s2 at nominally 80 Hz. For whole-body it is 1 m/s2 at nominally 16 Hz. Finally, for low-frequency, whole-body reference is 0.1 m/s2 at nominally 0.4 Hz. The duration of each test is 30 seconds for hand-arm, 1 minute for whole-body and 5 minutes for low-frequency whole body.

The 9200D and 9210D are each turnkey solutions with known NIST-traceable vibration display that can be used to conduct the hand-arm and whole-body elements of this test clause. The reference frequency for low-frequency whole-body is lower than the minimum frequency the shakers can achieve (0.7 Hz). But they are the only portable shakers that even come close to meeting the low-frequency whole-body requirement. The only mechanical option that can achieve 0.4 Hz is a long stroke shaker such as the 2129E025 paired with The Modal Shop’s 9155 Calibration Workstation. This system is designed and intended for laboratory use rather than portable field applications.

Mechanical frequency response tests are required at the center frequencies of the octave bands. For hand-arm applications the test range is 8 Hz to 2000 Hz. For whole-body the range is 0.5 Hz to 160 Hz. For low-frequency whole-body the range is 0.4 and 0.5 Hz. For verification testing one frequency weighting within each application must be tested with both mechanical and electrical signals. Users are given the option of testing the remaining weightings using mechanical or electrical signals but are not required to do both. When mechanically testing, the accelerometer shall be mounted in accordance with ISO 16063-21.

The hand-arm weighting and most of the whole-body weightings can be tested mechanically with either Mdel 9200D or 9210D Low Frequency Portable Calibrators.

ISO 8041 requires mechanical testing to be done before and after measurements are taken by the hygienist at the following points:

Hand-arm: 10 m/s2 at nominally 80 Hz and 10 m/s2 at nominally 160 Hz

Whole-body: 1 m/s2 at nominally 16 Hz

Low-frequency whole-body: 0.1 m/s2 at nominally 0.4 Hz

The user is required to record the field calibration results and any details regarding the settings within the vibration meter. This includes the initial sensitivity of the meter. Technicians should document the original sensitivity setting and any adjustments to this setting as changing this value based upon actual sensitivity of the transducer will greatly improve accuracy.

Model 9210D makes in-situ checks very simple for the technician because it has an accelerometer sensitivity display and ICP® power. Hence if the measured value on the meter is incorrect users can quickly determine the sensitivity of the transducer itself and, if necessary, adjust the input sensitivity on the meter to match. The 9210D and 9200D meet the frequency and amplitude requirements for hand-arm and whole-body. The low-frequency whole-body requirement is a lower frequency than either device is capable.

Currently no field calibrators exist that are capable of meeting the low-frequency whole-body in-situ check requirements (0.1 m/s2 at nominally 0.4 Hz). The ISO 8041 standard acknowledges this, stating “it is recognized that field calibrators are not currently available at such low frequencies.” But the standard does not offer a compromise to what is commercially available, instead stressing the importance of the aforementioned test point. The standard has been in existence for more than a decade and to date no manufacturer has been able to meet the low-frequency whole-body test point requirement. Models 9210D and 9200D have 0.7 Hz low frequency response and can generate 0.1 m/s2. They are the first and only portable devices to come within 0.3 Hz of meeting the requirements.

The 9210D and 9200D meet all other specifications within Annex A for distortion, load and transverse vibration. They are supplied with calibration certificates per ISO 16063-21 as desired by ISO 8041.